In summary, a reflective optical beam smoke detector system has a detector unit, which includes both a transmitter and a receiver, and a retro-reflector. The detector unit and the reflector are placed opposite each other at opposing ends of a volume to be protected and monitored. The transmitter projects a modulated beam, in this example a modulated Infrared (IR) beam, on to the retro-reflector which reflects the IR beam along the same axis back to the receiver. Smoke in the beam path will reduce the amount of light returning to the receiver. The receiver continuously monitors the amount of light received and, if it drops below a certain user-defined threshold, then an alarm is initiated. Typically, the beam is not continuously projected, but only once per second for a very short time ˜10 milliseconds (ms).
Installing two such detectors in an opposite manner comes with both advantages and disadvantages. This configuration is advantageous if the volume which is to be protected is longer than the specified protection distance of a detector. In that situation, which is not uncommon, if the distance is anything larger, an installer will install a detector on each wall, and install reflectors back-to-back at an approximate midway point between them. The advantage of this is that all of the electrical wiring to power supplies and fire panels remains at the edges of the building, rather than having to run such wiring to midpoints in a volume to be protected. Unfortunately, although there are advantages to this configuration, it is also the cause of problems, as the beam from one detector will fall upon the opposite detector unit. This scenario is exemplified in FIG. 1.
FIG. 1 illustrates a prior art detector system, identified generally by reference 100, which includes a first detector unit 110 and a second detector unit 120. In this example, the detector units are operating with Infrared (IR) light. Detector unit 110 includes both a transmitter and a receiver within the unit 110 and, as for detector unit 120, it also includes a transmitter and receiver within the unit 120. Each of the units 110; 120 is located in an opposed manner (as shown in FIG. 1) and have a corresponding reflector 111; 121, respectively, located within a volume to be monitored at a location some distance from the units 110; 120 and towards which a beam is projected by the detector units 110; 120. The corresponding reflectors 111; 121 are neither provided exactly equidistant nor centrally between the two detectors units 110; 120, although this is not an unusual configuration. Each unit 110; 120 operates independently of the other, such that if obscuration of the beam occurs between detector unit 110 and corresponding reflector 111, and between detector 120 and corresponding reflector 121, a warning or alarm is independently signalled or activated. Of course, if obscuration occurs in both regions, then two alarms would be signalled or activated. There are, of course, advantages or reasons as to why it would be useful or necessary to locate detector units 110; 120 in an opposed manner, even though there is a chance, or even likelihood, that some of the beam projected from one might interfere with detection of the beam of the other unit.
Using the beam 122 from detector unit 120 as an example, and as shown in FIG. 1 in particular, normal detection can occur between the unit 120 and its reflector 121, as per normal operation of the beam detector. However, in relation to part 122′ of the beam 122 which projects past the reflector 121 and illuminates the environs of the detector 110, as indicated by illuminated regions 101 and 102, and a corresponding shadow region 103 caused by the reflector 121 in the beam 122, if detector unit 120 is projecting its beam 122 at a same time as detector unit 110 is expecting to receive its own projected beam (not shown), there is a strong chance that detector unit 110 will receive more light than expected, some of which from beam part 122′.
As the detector units 110; 120 only transmit for 10ms every second, this does not cause a continuous problem as the two units will often transmit at different times. However, owing to timing differences between the two detectors, they will eventually come into phase where they will transmit at the same time. Each detector will not only receive light from its own reflector but also light from the opposite transmitter. As the beams are modulated, depending upon how the peaks and troughs of the two beams coincide, this will cause an increase or decrease in the signal strength. The effect of this is that the effective signal strength received may go up or down at random, which leads to false alarms and the signalling of faults. Further, the phasing in and phasing out again of the beam transmissions can be somewhat random also, for example, the beams could be out of phase for hours, days or even weeks but once in phase they could be in this situation for minutes, hours or days, so it is hard to predict how long the beams will be in phase.
It is also known to place a baffle between a set of back-to-back reflectors, so as to increase the size of a shadow region caused by the light of opposed detector units, such that receiving units per se are within that shadow region. Such baffles are necessarily relatively large when compared to the reflectors and rather unsightly. The present invention is aimed at alleviating the above disadvantage(s) associated with beam detectors.